A one-dimensional transient ground heat exchanger model is proposed to account for fluid and grout thermal capacities in borehole ground heat exchangers with the objective of predicting the outlet fluid temperature for varying inlet temperature and flow rate. The standard two-pipe configuration is replaced with an equivalent geometry consisting of a single pipe and a cylinder core filled with grout. Transient radial heat transfer in the grout is solved numerically while the ground outside the borehole is treated analytically using the cylindrical heat source method. The proposed model is validated successfully against analytical solutions, experimental data, a three-dimensional transient numerical model, and TRNSYS's Type 451. For a typical two-pipe configuration, it is shown that the fluid outlet temperature predicted with and without borehole thermal capacity differs by 1.4, 0.35, and 0.23 degrees C after 0.1, 0.2 and 1 h, respectively. Annual simulations are also performed over an entire heating season (5600 h) with a 6 min time step. Results show that the outlet fluid temperature is always higher when borehole thermal capacity is included. Furthermore, the difference in fluid outlet temperature prediction with and without borehole thermal capacity increases when the heat pump operates infrequently. The end result is that the annual COP predicted is approximately 4.5% higher when borehole thermal capacity is included. (C) 2013 Elsevier B.V. All rights reserved.